KEY WEST, Fla. – Hurricane Irma barreled toward the U.S. mainland Wednesday, prompting the southernmost county in Florida to begin evacuations along its lone island-hopping highway while the “potentially catastrophic Category 5″ storm menaced Puerto Rico and a wide swath of the Caribbean.

Forecasters said Irma posed an increasing threat to South Florida, a sprawling and densely populated mass of cities and suburbs hugging the coastline. As dire warnings mounted, schools and offices began to shut down, grocery store shelves were wiped clean and authorities ordered evacuations with more to follow.

The most powerful hurricane to threaten the Atlantic coast in more than a decade, Irma has swelled into a monster force with maximum sustained winds near 185 mph, according to the National Hurricane Center. The center said Irma’s “extremely dangerous core” would move over the Leeward Islands on Wednesday morning before heading toward the northern Virgin Islands and Puerto Rico later in the day.

Throughout the American territories and other Caribbean islands in Irma’s path, residents watched the storm with fear, wondering whether they would emerge from Irma with destroyed homes or no electricity for months. Irma’s eye passed over Barbuda at around 1:47 a.m. Wednesday, the National Weather Service said, while on the French Caribbean islands of St. Martin and St. Barthelemy, residents were ordered to remain indoors.

According to the Post’s Capital Weather Gang, Barbuda took a direct hit and the weather station there registered a wind gust of 155 mph hour before going offline, while the storm surge on the island – or the swell of water above normally dry land – reached at least 8 feet. By Wednesday morning, the National Hurricane Center said Irma’s eye had passed over St. Martin and warned that the storm could bring dangers including life-threatening storm surges, destructive winds, flash floods and mud slides to Cuba, Haiti, the Dominican Republic, the Bahamas and beyond.

Further north, the Florida Keys on Wednesday morning ordered the first mandatory evacuations triggered by Irma in the United States. Monroe County, which includes the Keys and covers the southernmost stretch of the Sunshine State, began mandatory evacuations of tourists and visitors on Wednesday morning. The county is also home to some 80,000 residents, who were ordered to evacuate beginning Wednesday evening.

In Key West, hotels closed down ahead of the evacuation order and the airport was scheduled to halt operations later Wednesday. Gas stations reported low fuel stocks and grocery stores ran out of bottled water. Residents and business owners boarded up windows and hauled boats out of the water, while tourists and residents had already begun crowding the single highway that snakes through the 120-mile island chain and into the Florida mainland.

Many businesses on Key West’s famed Duval Street were shuttered Tuesday – with the exception of a few bars and restaurants – and many residents were streaming to the mainland by car on Route A1A.

“We’re emphatically telling people you must evacuate,” said the director of Monroe County’s Emergency Operations Center, Martin Senterfitt. “You cannot afford to stay on an island with a Category 5 hurricane coming at you.”

Carolyn Boutte, 44, said she and her husband moved to a house in Key West four years ago from Gloucester, Mass., and they have never been through a hurricane threat like this. They searched for gas on Tuesday so they could escape, but the first three stations already had run out of fuel and lines were long everywhere else. She finally ran into some luck – at a station where she had to wait 45 minutes for a fill.

“My husband and I are packing up the dog and our Harley Davidson,” said Boutte, a marine biologist. “Unless the hurricane changes paths, we are getting out of here in the next couple of days.”

Even as the full devastation from Hurricane Harvey was still being tallied in Texas and along the Gulf Coast, authorities have shifted their attention to Florida, with a particular unease in South Florida, home to 6 million people across Miami-Dade, Broward and Palm Beach counties – many of whom vividly remember Hurricane Andrew’s onslaught a quarter-century ago.

The National Weather Service said Wednesday that the threat of an impact to South Florida “continues to increase,” with concerns about what could happen between Friday night and Monday.

President Donald Trump on Tuesday evening declared an emergency in Florida as well as the U.S. Virgin Islands and Puerto Rico, and Florida Gov. Scott, R, activated 100 members of the Florida National Guard and said he has directed all 7,000 members to report for duty on Friday.

The Pentagon approved the use of two ships – originally deployed for Harvey relief – to assist in Florida as needed. William “Brock” Long, administrator of the Federal Emergency Management Agency, said that incident management assistance personnel already are on the ground in vulnerable areas.

“Just like in Texas, the response to Irma is going to take all levels of government and the whole community,” Long said in a statement. “This has the potential to be a catastrophic storm.”

Officials across Florida responded to the dire forecasts by slowly shutting down the contours of daily life. Monroe County canceled classes for the rest of the week, while school districts in Miami-Dade, Broward and Palm Beach – three of the country’s largest, with a combined enrollment north of 800,000 students – said they were canceling classes on Thursday and Friday.

The NFL said the Miami Dolphins season opener scheduled for Sunday afternoon against Tampa Bay would not be played in South Florida as planned, and would move either to a neutral location the same day or would be rescheduled for later in the season. The University of Central Florida in Orlando, which could face punishing weather if Irma crawls up the coastline, moved a football game to Friday night, while other schools said they were still monitoring the situation.

Scott, who earlier this week declared a statewide emergency, has warned that Irma could require large-scale evacuations or severely impact areas battered last year by Hurricane Matthew, which sent punishing flooding into parts of the state.

But it was still not clear Wednesday how much of the state could be imperiled by Irma. The uncertainty of Irma’s track and the geography of the Florida peninsula combined to create an unusually broad, essentially statewide sense of emergency in a place where most of the population lives along the coasts. Irma could potentially ride up either side of Florida or track further west into the Gulf of Mexico and endanger the state’s panhandle.

Miami-Dade, the most populous county in Florida, said it would begin ordering evacuations on Wednesday, likely starting with the coastal regions and Miami Beach.

In Estero, on the state’s Gulf Coast, residents were either hunkering down or starting to flee. Stocks of water and flashlights at grocery stores were wiped out, and gas was becoming scarce. A sign on the door of a Speedway gas station warned customers: “No gas, no propane, no water, sorry.”

“We’ve never been this worried in our entire lives,” said Jose Torres, 25, who plans to evacuate to Georgia on Wednesday.

The main routes out of South Florida are Interstate 95 and the Florida Turnpike, which can be prone to traffic on the best of days. In an effort to smooth travel, Scott on Tuesday ordered that no tolls be collected.

The main route west, meanwhile, is Interstate 75 – the so-called “Alligator Alley.” If the storm does track up the state’s spine, as Scott noted, that could make evacuations extremely complex.

The last major hurricane – registering as a Category 3 storm or stronger – to make landfall in Florida was Hurricane Wilma in October 2005. Wilma also was the last major hurricane to make landfall in the United States until Harvey struck Texas late last month.

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